Electron beams have been widely used in lithographic tools and/or microscopes in, for example, producing or obtaining images of structures of micro devices that are commonly found in semiconductor integrated circuits. In a typical apparatus or arrangement employing electron beams, a stream of electrons or charged particles may be emitted from or produced by a cathode. The stream of electrons may subsequently be accelerated to a desired velocity or energy level, by being subjected to a voltage difference or potential between the cathode and an anode, to propagate or travel along a column (“electron beam column”) as an electron beam. During propagating or traveling, the electron beam may be shaped, focused and/or deflected along one or more stages or sections or segments of the column. The electron beam may finally hit a target object of interest, such as a semiconductor micro device, provide a desired exposure to the target object, and cause to produce an image of the target object, as is well known in the art.
An electron beam column may include one or more center tubes. As is well known in the art, in order to meet stringent requirements for high precision in both shape and position of an electron beam, center tubes are generally required to be able to provide a high degree of vacuum environment for the electron beam that travels inside. In addition, the center tubes may need to be able to avoid or eliminate as much as possible charging build-up on the inner surfaces that are visible to the electron beam. Moreover, in areas or segments where dynamic magnetic fields are applied, the center tubes may also need to be able to prevent or suppress eddy current that may otherwise adversely affect the beam focus and trajectory. Additionally, it is highly desirable that stray electrons (from the electron beam) that strike inner surfaces of the center tubes do not scatter or produce secondary electrons that could further impede beam focus and stability.
Most electron beam systems or apparatuses to date use non-conductive (e.g., glass, ceramic and/or plastic) center tubes to contain and maintain a vacuum environment, with a thin conductive layer of coating being applied to the inner surfaces of the center tubes to prevent the build-up of charges. In order to achieve relatively good performance, it is generally required that the coating being applied to the inner surfaces be made of materials that, when being exposed to air or an oxygen-containing environment, do not transform into an oxide insulating layer; be continuous, pinhole free; free of magnetic material contaminants; and be able to absorb partially or most of primary and secondary electrons. In addition, in order to be able to hold eddy current 1/e time constant below 3 micro-second, as is known in the art as being a common requirement, the coating may have a thickness of up to around 12 microns. However, applying a coating of this level of thickness is typically being associated with a difficult and low yield process. Furthermore, in addition to the above technical challenges, none of the current electron beam systems or apparatuses has effectively addressed problems such as, for example, electron absorption and scattering.
In certain electron beam systems, center tubes made of solid metal may be used in segments of the electron beam column where dynamic magnetic fields are not applied. However in situations like this, a thin layer of coating of Au, for example, may be needed in order to prevent the inner surface of the solid metal center tube from forming an oxide layer by the virtue of being exposed to air from time to time. Coupled with great level of difficulty, low yield, high cost, and low reliability (especially for longer tubes), some solid metal center tubes were fabricated in the past but so far none has demonstrated the desired electron absorption characteristic. Also in the past, some electron beam systems have used carbon material which may be strategically placed inside the vacuum near the electron beam to mitigate the problem of excessive electron scattering.
Therefore, there is a need in the art to create a better solution that may address the problems of current electron beam systems or apparatuses as described above. For example, a center tube that may mitigate the above described problems may be highly desirable.